1. Field of the Invention
This invention generally relates to an information transmission system. This invention particularly relates to a system for encrypting a great amount of, for example, image information and transmitting the encryption-resultant information. This invention also relates to an information sending apparatus and an information receiving apparatus used in an information transmission system.
2. Description of the Related Art
In some of conventional information transmission systems, a sending side encrypts information before sending the encryption-resultant information toward a receiving side, and the receiving side decrypts the incoming information into the original information. The encryption is to protect the information against an illegal attack such as a tapping or altering action during the transmission thereof. Known encryption methods include first one called stream cipher, and second one utilizing authentication about a communication opposite party.
Japanese book entitled “Modern Cipher”, written by Tatsuaki Okamoto and Hirosuke Yamamoto, published by Sangyo-Tosho Kabushikikaisha on Jun. 30, 1997, pages 73-75, discloses stream cipher designed so that a sending side modulo-2-adds an information bit stream and a key bit stream on a bit-by-bit basis to get an encryption-resultant bit stream, and a receiving side modulo-2-adds the encryption-resultant bit stream and a key bit stream on a bit-by-bit basis to recover the original information bit stream. The key bit stream used by the receiving side is the same as that used by the sending side. In practice, the sending side and the receiving side generate a pseudo-random-number bit stream, and hold it as a common key bit stream.
The foregoing Japanese book, pages 151-162, also discloses password-based certification about a communication opposite party, secret key cryptography, public key cryptography, and digital-signature-based authentication.
According to the password-based certification, a user secretly has a password. The user sends the password to a center (an inspector or a verifier) when requesting an access to the center. In the center, a check is made as to whether or not the password is correct. When the password is correct, the user is proved to be legitimate. Then, user's access to the center is granted.
In the secret key cryptography, a user and an inspector (a verifier) have a common secret key. The user encrypts information in response to the secret key. The user sends the encryption-resultant information to the inspector. The inspector decrypts the incoming information in response to the secret key to recover the original information.
In the public key cryptography, each user or inspector has a pair of a private key and a public key corresponding to the private key. An inspector encrypts information in response to a public key of a user. The inspector sends the encryption-resultant information to the user. The user decrypts the incoming information in response to user's private key. The user sends the decryption-resultant information to the inspector. The inspector compares the incoming information with the original information to decide whether or not the user has a correct private key corresponding to user's public key, that is, whether or not the user is legitimate.
Generally, digital signatures rely on public key cryptography. According to the digital-signature-based authentication, an inspector sends information to a user. The user scrambles the incoming information in response to user's private key to generate a digital signature. The user sends the digital signature to the inspector. The inspector descrambles the digital signature in response to user's public key. The inspector collates the result of the descrambling with the original information to check whether or not the digital signature is correct, that is, whether or not the user is legitimate.
The above-mentioned cryptography and authentication are based on the difficulties in solving particular mathematical problems such as integer prime-factorization problems or discrete logarithm problems.
Typical information transmission systems include a combination of hardware and software (that is, a computer and a program) for implementing encryption and decryption. In the typical systems, a great part of encryption and decryption work is assigned to the software so that a time taken to complete the work tends to be long. Accordingly, the typical systems tend to take a long time to perform certification or authentication.